Photoconductive element exhibiting photoconductive dichroism and process of using same

ABSTRACT

AN ELECTROPHOTOGRAPHIC PROCESS FOR THE PRODUCTION OF REFLEX COPIES IN WHICH A DOCUMENT IS POSITIONED ADJACENT A PHOTOCONDUCTIVE ELEMENT WHICH EXHIBITS PHOTOCONDUCTIVE DICHROISM AND HAS A PREFERRED ABSORPTION AXIS, AND IN WHICH THE PHOTOCONDUCTIVE ELEMENT IS UNIFORMLY EXPOSED THROUGH THE PHOTOCONDUCTIVE ELEMENT WITH POLARIZED LIGHT WHOSE VECTOR, RELATIVE TO THE ABSORPTION AXIS, IS SUCH THAT THE LIGHT IS NOT ABSORBED. THE POLARIZED LIGHT IN STRIKING THE DOCUMENT IS ABSORBED IN SOME AREAS, NORMALLY THE DARK IMAGE AREAS, AND DEPOLARIZED AND REFLECTED IN OTHERS, NORMALLY THE LIGHT BACKGROUND AREAS. THE LIGHT FROM THE REFLECTED AREA, BEING DEPOLARIZED, CONTAINS LIGHT WITH AN ELECTRIC VECTOR WHICH WILL BE ABSORBED BY THE PHOTOCONDUCTIVE ELEMENT AND THE ELEMENT IS THUS EXPOSED TO A PATTERN CORRESPONDING TO THE PATTERN OF THE DOCUMENT. THIS RENDERS THE PHOTOCONDUCTIVE ELEMENT CONDUCTIVE AND CAPABLE OF TRANSPORTING AN ELECTROSTATIC CHARGE AND, HENCE, PERMITS THE FORMATION OF AN ELECTROSTATIC CHARGE PATTERN CORRESPONDING TO THE DOCUMENT.

United States Patent Olfice 3,598,582 Patented Aug. 10, 1971 US. Cl.96-1.5 24 Claims ABSTRACT OF THE DISCLOSURE An electrophotographicprocess for the production of reflex copies in which a document ispositioned adjacent a photoconductive element which exhibitsphotoconductive dichroism and has a preferred absorption axis, and inwhich the photoconductive element is uniformly exposed through thephotoconductive element with polarized light whose vector, relative tothe absorption axis, is such that the light is not absorbed. Thepolarized light in striking the document is absorbed in some areas,normally the dark image areas, and depolarized and reflected in others,normally the light background areas. The light from the reflected areas,being depolarized, contains light with an electric vector which will beabsorbed by the photoconductive element and the element is thus exposedto a pattern corresponding to the pattern of the document. This rendersthe photoconductive element conductive and capable of transporting anelectrostatic charge and, hence, permits the formation of anelectrostatic charge pattern corresponding to the document.

BACKGROUND OF THE INVENTION Field of invention This invention relates ingeneral to contact reflex reproduction and in particular toelectrophotography with a contact reflex exposure of a photoconductiveelement.

Description of prior art In electrophotographic processes, of whichso-called xerography is a single example, an element comprising aphotoconductive insulator is uniformly electrostatically charged andexposed to a pattern of light to cause the formation of electrostaticcharge patterns on the photoconductive element. The thus-formedelectrostatic pattern is then developed with an electroscopic powdereither while it still is on the photoconductive element or, for example,after it has been transferred to another surface sheet. In the former(xerographic) process, the developed electrostatic charge pattern issubsequently transferred to paper and the photoconductive element iscleaned to remove any non-transferred powder.

In general, in the above described exposure step, graphic informationhas normally been transferred from original to photoconductive elementby employing lenses or similar optical systems with the result that thephotoconductive element is exposed to a pattern of ligh and dark,corresponding o the graphic information on the original. With opticalgraphic information transfer, a substantial part of the cost of theelectrophotographic apparatus is attributable to the optical system.Moreover, a relatively large and bulky housing must be provided tosupport not only the optical system but to provide, as well,predetermined distances between the optical system and the originaldepending on the focal length of the optical system. Further, opticalsystems are very ineflicient in their utilization of available lightand, therefore, some photoconductors would require too high an exposuretime in an optical system employing a conventional relativelyinexpensive light source so that more powerful and more expensive lightsources must be employed.

In lieu of an optical information transfer, there is an exposuretechnique known as contact reflex. With this type of graphic informationtransfer, the original to be copied is brought into contact with thephotosensitive element; the sandwich thus formed is exposed to lightwhich passes first through the back of the photosensitive element. Thelight passing through the photoconductive element and striking theoriginal is absorbed by the dark areas of the original and reflectedback from the white or light areas of the original, forming an exposurepattern on the photosensitive element.

There are numerous reasons why this type of exposure has not achievedcommercial acceptance in electrophotography. It is clear that for usefuldegrees of light modulation to occur, the element must freely pass lightto the surface of the original. Commercially used photoconductors, suchas selenium, are essentially opaque to the exposing radiation. In orderto secure adequate transmission of light to the original through theelement, it has been necessary to construct photoconductive elementshaving elaborate and expensive screen type configurations. In addition,when these commercially used photoconductors are employed in thin layersto render them transparent, the resulting voltage difference between theareas of photoconductor exposed by the reflected radiation relative tothe areas of the photoconductor only exposed by the uniform exposure isnot suflicient to achieve high quality copies of the original.

One of the key drawbacks then to contact reflex methods is the fact thatcontrast is degraded owing to the neces sary pre-exposure of the entiresurface to non-information bearing light, which pre-exposure isintrinsic to all reflex methods.

A non-electrophotographic contact reflex exposure process has beendescribed in which this drawback is overcome because the photoprintingmaterial contains an oriented dichroic light sensitive diazo compound.For the exposure step, the original to be copied is brought into contactwith the photoprinting material and the sandwich thus formed isuniformly exposed through the photoprinting material. The uniformincoming light, however, is plane polarized with its electric vectornormal to the principal transition moment of the dichroic lightsensitive diazo compound. Decomposition of the diazo compound isproduced only by absorbed light. Since the uniform incoming light is notabsorbed due to the lack of parallelism between the electric vector ofthe plane polarized light and the absorption axis of the dichroic diazocompound, no photo decomposition occurs. However, upon reflection by thesurface of the original, the polarized light becomes substantiallydepolarized (e.g., no preferred electric vector direction exists) inbeing reflected from the white areas of the original back to thephotoprinting material, and since a component of electric vector nowexists in the direction of the transition moment of the dichroic diazoit is now substantially absorbed by the diazo compound therebydecomposing the diazo compound to form a developable image correspondingto the original image. Such substantial depolarization as describedalways results when plane polarized light is reflected from thediffusely reflecting surface, such as paper.

While this non-electrophotographic process teaches an important stepforward in the contact reflex exposure art, no commercial use appears tohave been made of the process most probably because the photoprintingmaterial can only be used a single time. The cost of manufacturing anoriented photoprinting material useful for making a copy of only asingle, particular original is too great to make it practically useful.

3 SUMMARY OF THE INVENTION Accordingly, it is an object of the inventionto provide a novel reproduction process which is relatively inexpensiveto implement into a compact reproduction device and capable, because ofthe high light-utilizing efficiency of the contact copying mode, ofyielding high quality copies at a very high rate of speed.

It is a more detailed object of the present invention to provide acontact reflex exposure process using polarized light in which thephotosensitive element is capable of being reused repeatedly in thereproduction of copies, thereby spreading the cost burden of the elementover thousands of copies.

Still another object of the invention is to provide a novel reusablephotoconductive element which is capable of use in electrophotographicreproduction processes with contact reflex exposure using polarizedlight and which produces high quality copies in such reproductionprocesses.

A further object of the present invention is to provide a novelphotoconductive element which is well adapted to copying half toneimages of extended area with high quality, even with differentialvoltage development methods such as the so-called cascade developingmethod.

In general, the foregoing objects are achieved by a novel reusablephotoconductive element comprising an oriented dichroic material whichis capable of transmitting essentially all of the polarized light havinga direction of electric vibration such that the photoconductive elementdoes not absorb the light and hence does not become conductive whenexposed to such polarized light. Thus, the photoconductive element canbe brought into contact with a document and uniformly exposed to suchpolarized light and the light is efficiently transmitted to the surfaceof the document without the photoconductive element becomingphotoconductive as the light initially passes through the element.However, a substantial portion of the light reflected back from thelight reflecting and depolarizing areas of the document is absorbed,thereby resulting in a high contrast latent image in the photoconductiveelement corresponding to the image of the document. This latent image isthe basis for an electrostatic image which can be developed in any ofthe conventional ways to provide a copy of the document. Because theelement is a photoconductor, it can be used repeatedly to provide copiesof the same or different documents.

It is not necessary that the reflected light be completely depolarized,but only that, following reflection, it contains components of electricvibration which will be absorbed by the photoconductive element; forexample, components of electric vibration normal to the polarized lighttransmitted through the element.

Other and further objects and features of the invention will be apparentin the following more particular description of the preferred embodimentof the invention.

Absorption or emission of light in a single molecule or a cooperatingarray of molecules is qualitatively ascribable to a motion of electriccharge within the molecule or cooperating molecular array. Lightgenerated by the motion of electric charge within a molecule ischaracterized by an electric field disturbance (the electric vector)whose direction coincides precisely with the direction of motion of theelectric charge in the molecule giving rise to the light. Conversely,light absorption requires that the incident light possess a component ofelectric field disturbance in the direction of the electric moment ofthe molecule. The magnitude and direction of the electric charge motionwithin the molecule is measured by the socalled transition moment of themolecule and governs the intensity of the absorption and emission oflight. In general, a molecule is characterized by three mutuallyperpendicular transition moments which relate directly to the capabilityof the molecule to interact with light. The three transition moments,however, may vary considerably in magnitude and, in particular, onemoment may be very large relative to the remaining two. If a largenumber of such molecules are aligned (oriented) into an (uniaxiallycrystalline) structure so that the major transition moments are allparallel for all the molecules, the components of electric field ofnatural light incident on the array which are parallel to the alignedelectric moments of the array are preferentially absorbed with theresult that the remaining light is plan polarized, i.e.light is producedwhose electric vector vibrates in a single well-defined direction. Theforegoing is the basis for the so-called dichroic polarizer asexemplified by a Polaroid sheet. It will be understood that thisexplanation as to molecules is equally applicable to crystals.

A material then is dichroic or exhibits dichroism if the absorption oflinearly polarized light varies in accordance with the direction of theelectric vector. Stated another way, a strongly dichroic material is onewhich will transmit essentially all of the linearly polarized lighthaving an electric vector normal to its absorption axis while stronglyabsorbing the linearly polarized light whose electric vector is parallelto its absorption axis. It should be evident that, if linearly polarizedlight encounters an oriented dichroic array whose principal transitionmoment or absorption axis is normal to the direction of the electricvector of the light, no light can be absorbed by the oriented dichroicarray, e.g.such light will be without substantial effect of any kind onthe oriented dichroic array.

In general, the photoconductive element of this invention can beembodied in a number of ways, the preferred of which is by employing oneof the novel photoconductors of US. Pats. 3,489,558 and 3,501,293, whichexhibits dichroism when uniaxially oriented. Such photoconductors may beused either alone as the photoconductive material or in association withanother material which is capable of transporting charge carriers andwhich is transparent to the wavelengths of polarized light employed.This material may be either insulating or semiconducting so long as itsresistivity is sufiiciently high so that the composite photoconductiveelement will retain a charge on its surface in the dark. Preferably, theresistivity of the material is 10 ohm-cm. or greater. Thus, this secondmaterial is inactive until light is absorbed by the dichroicphotoconductor.

Another embodiment for the photoconductive element of the invention isto use a photoconductor, again which is transparent to the wavelengthsof polarized light, in conjunction with an oriented dichroic activatoror sensitizer, which coacts with a photoconductor in some manner such asthrough the formation of a charge transfer complex. With thisembodiment, the photoconductor is photoconductively inactvie until thedichroic activator or sensitizer absorbs light and transfers theabsorbed energy to the photoconductor.

Regardless of the embodiment of the photoconductive element employed,the dichroism of the element is either due to the crystalline form orthe molecular form of the dichroic material. In either case from apractical standpoint of manufacturing a dichroic material of a largearea or sheet size, essentially all of the crystals or molecules must bealigned so that they act as a single dichroic unit, i.e., haveessentially the same preferred absorption axis. With crystals, this canbe accomplished by dispersing the crystals in a stretchable sheet suchas a sheet of polyvinyl alcohol, and stretching the sheet in anunidirection. Of course, the stretchable sheet must be essentiallytransparent to the Wave lengths of light which the dichroic materialabsorbs. In addition, the crystals should be microcrystalline in size tominimize light scattering.

Dichroic molecules can be aligned in a number of ways, such as-(a)stretching as previously described, (b) attaching the moleculeschemically within. a homogeneous material that already has a high degreeof orientation in an unidirection, (c) coating the molecules onto thesurface of a sheet which already has a preferred direction oforientation, (d) coating them on a surface by undirectional rubbingwhich may be followed by stretching, and extruding.

Normally, the extent that a dichroic sheet prepared by one of the abovemethods exhibits dichroism is measured by its optical dichroic ratio, RThis ratio, R is defined as d /d wherein d is the optical densityobtained when the incident light is linearly polarized with thedirection of the electric vector parallel to the transition moment axisfor maximum absorption and wherein d is the optical density obtainedwhen the incident light is linearly polarized with the direction of theelectric vector perpendicular to the axis for minimum absorption. Itwill be apparent that dichroic sheets with high dichroic ratios,especially over a wide bandwidth, are desired.

While this optical dichroic ratio is an indication of the usefulness ofa particular dichroic material in the photoconductive element of thepresent invention, a dichroic photodecay ratio is more accurate and moreimportant in determining the suitability of a particular material. Thisratio, R is defined as 12 /11 wherein p is the decay rate of anelectrostatic charge on the photoconductive element when the incidentlight is linearly polarized having the electric vector for maximumabsorption and wherein p, is the decay rate obtained when the incidentpolarized light has the electric vector for a minimum absorption. Thesetwo rates may be based either on the initial decay rate or the decay atT (the exposure time required to reach one-half of the originalelectrostatic potential). This ratio is a measure of the usefulness ofthe oriented dichroic material of the present invention.

In general, for producing good quality copies of the original, thedichroic photodecay ratio should be at least greater than 2 andpreferably should be greater than 5. It should be recognized that thedichroic photodecay ratio can be varied by changing the concentration ofthe dichroic material and will vary depending upon the method offabrication of the element and its final configuration. The ratio, R canbe increased by activating or sensitizing a dichroic photoconductivematerial so as to increase 1ts photoconductivity through the formationof a charge transfer complex or some other mechanism. Likewise, in theembodiment of the photoconductive element employing a chargetransporting material insensitive to the wave lengths of polarizedlight, it too can be activated or sensitized as long as it is notrendered sensitive to the wave lengths of polarized light.

With the above general description of the photoconductive element of thepresent invention, the following describes the use of such an element inthe electrophotographic process of the present invention. Thephotoconductive element is uniformly electrostatically charged,following which a document to be reproduced is brought into contact withthe charged surface of the photoconductive element. Next, the freesurface of the photoconductive element is exposed to polarized lighthaving its electric vector so oriented with respect to the absorptionaxis of the ele ment that there is maximum transmittance of the lightthrough the dichroic photoconductive element. Upon striking theoriginal, the polarized light is essentially absorbed in the dark orblack areas, usually the print areas, and depolarized in the light orwhite areas, normally background. Such depolarized light is reflectedback to the photoconductive element and a sufficient portion of it hasan electric vector normal to the vector of the originally transmittedpolarized light and parallel to the absorption axis of the dichroicmaterial that suflicient light is adsorbed by the photoconductiveelement to cause selective dissipation of electrostatic charge and theformation of a charge pattern corresponding to the pattern of thedocument. Now, the electrostatic charge pattern can be developed withtoner in one of the known electrophotographic ways, such as cascade,magnetic brush or fur brush, and the developed pattern transferred topaper to provide 6 a high quality copy of the document. Alternatively,the electrostatic charge pattern can be transferred to a dielectricsurface and developed thereon. After being cleaned, if the electrostaticcharge pattern is developed on the photoconductive element, thephotoconductive element is ready for additional cycles from which resultadditional high quality reproductions of the same or differentdocuments.

-It will be understood that document includes not only those havingareas which without further intervention depolarize and reflect thelight back into the photoconductive element but also includes printedtransparencies or translucencies which have been backed by adepolarizing and reflecting element.

The preferred embodiment of the photoconductive element of the presentinvention comprises a transparent conductive substrate, such as a layerof cellulose triacetate having an aluminized surface, carrying anoriented dichroic photoconductor, such as2,6-bis-[p-dimethylaminocinnamyldeneamino] benzo 1,2-d 4,5 -d'bisthiazole described in US. Pat. 3,501,293. Herein, the dichroicphotoconductor is rubbed on in dry powder form in an unidirection whichestablishes a preferred axis for maximum absorption when the electricvector of the polarized light is parallel to the axis and a minimumabsorption or maximum transmission of the light when its electric vectoris perpendicular to the axis. On top of the oriented dichroicphotoconductor is a layer of a transparent normally insulating materialwhich is essentially insensitive to the wave lengths of polarized light,but capable of transporting charges generated by the dichroicphotoconductor. Herein, the layer comprises poly-N-vinylcarbazole whichis essentially insensitive to the visible light to be used for theexposure of the photoconductive element.

With the surface of the tarnsparent poly-N-vinylcarbazole layer carryinga uniform electrostatic charge, the photoconductive element is exposedto visible polarized light from, for example, an unpolarizedincandescent light source fitted with a light polarizing sheet, such asa H- sheet polarizer (manufactured by Polaroid Corporation). Thephotoconductor element is oriented relative to the "polarizer such thatthe electrical vector of the polarized light is prependicular to theabsorption axis of the dichroic photoconductor. Thus, there is maximumtransmittance of the polarized light through the oriented dichroicphotoconductor and, hence, through the photoconductive ele ment as thelight strikes the substrate side of the element.

With a document in contact with the charged surface of thephotoconductive element during this exposure, the polarized light isessentially depolarized in the light colored or White areas of thedocument and is reflected back to the photoconductive element. Thistime, however, the electric vibrations of up to one-half of thedepolarized light are parallel with the absorption axis of the dichroicphotoconductor so that up to one-half of the reflected light may beabsorbed. The areas of the dichroic photoconductor absorbing the lightbecome conductive and, it is believed, generate charge carriers whichare transported by the normally insulating layer. Regardless of thetheory, the result is that the charge on the transparent photoconductoris dissipated so that an electrostatic charge pattern is formedcorresponding to the pattern of the document. Of course, the polarizedlight striking the dark or black areas of the document is essentiallyabsorbed and not reflected so that charge remains in such areas. Theelectrostatic charge pattern can be developed by one of the conventionaltechniques.

Another embodiment of the photoconductive element of the presentinvention is one in which the positions of the transparentphotoconductive layer and the dichroic photoconductive layer arereversed so that the transparent layer is adjacent the aluminizedsubstrate. Because the dichroic photoconductive layer now forms the toplayer, it is desirable in those cases in which this layer is easilyabraded, to overcoat the dichroic layer with a protective layer, such asa transparent insulating material of, for example, cellulose acetate, orany other transparent material which will be sufliciently insulating tohold an electrostatic charge and will not be easily abraded.

A third embodiment employing a dichroic photoconductor comprises asingle layer carried on a transparent conductive substrate. Herein, inthis embodiment, the dichroic photoconductor, such as one of thosedescribed in US. Pat. 3,489,558, is dispersed in an insulatingtransparent matrix, such as polyvinyl formal. To orient the dichroicphotoconductor, the matrix is stretched in a unidirection prior toapplying it, for example, by lamination, to the conductive substrate,such as aluminized cellulose triacetate. If desired, the insulating filmmay also be photoconductive as long as the film is essentiallyinsensitive to, or not rendered photoconductive by virtue oftransmitting, the wave lengths of polarized light used for exposure.

To increase the photoconductivity of one of the abovedescribedphotoconductive elements, there is incorporated in the element either adye sensitizer or an activator which is also known as an electronacceptor or, in some cases, when the photoconductor is an electronacceptor, an electron donor. Examples of such dye sensitizers andactivators are set forth in U.S. Pats. 3,037,861, 3,169,060 and3,287,113. In addition, if it is desired to have the photoconductiveelement exhibit persistent conductivity, the dye sensitizer andactivator combinations described in U.S. application Ser. No. 474,977,filed July 26, 1965, may be used in the preparation of suchphotoconductive elements.

If the embodiment of the photoconductive element is one in which acharge transfer complex is formed, either with a dichroic photoconductorand an activator, or a photoconductor and a dichroic activator, and thecomplex acts as a dichroic entity, then the absorption spectrum of thecomplex should be within or essentially match the wave lengths ofpolarized light employed. If a complex is formed but is not a dichroicentity, the non-dichroic activator or non-dichroic photoconductor mustabsorb essentially outside the wave lengths of the polarized lightemployed. The same is true for a non-dichroic dye sensitizer. When thephotoconductive element comprises one of the embodiments which includesa charge transporting layer, the dye sensitizer or activator added tothis layer must absorb essentially outside the wave lengths of polarizedlight being employed.

Except for the charge transfer embodiment, all of the previousembodiments described photoconductive elements in which the dichroicmaterial is in itself a photoconductor. The following embodiments of thepresent invention are directed to a photoconductive element in which thedichroic material serves as a sensitizer or activator for thephotoconductor which is essentially insensitive or non-photoconductivein the wave lengths of polarized light employed for exposure.

The first of these embodiments comprises a transparent conductivesubstrate on which is applied a polymeric layer having a direction oforientation due to, for example, unidirectional stretching. Thispolymeric layer is stained with a material such as iodine which causesthe polymer to exhibit dichroism and also serves as an activator for aphotoconductive layer applied over the polymeric layer. Thisphotoconductive layer is essentially transparent to the wave lengths ofpolarized light to be employed. In addition, the activator must besufliciently abundant at the interface of the polymeric layer and thephotoconductive layer to activate or render the latter layer conductivewhen light is absorbed by the polymeric layer.

If desired, the two above-described layers may be reversed so that thepolymeric layer is uppermost. However, inasmuch as the polymeric layermust be capable of holding an electrostatic charge in the dark on itssurface, it must have a dark resistivity of at least 10 ohm-cm. andpreferably should be in the range of 10 to 10 ohm-cm. Thus, somepolymeric materials, such as polyvinyl alcohol, may not be suited forthis embodiment, or may have to be overcoated with an insulatingmaterial to achieve the proper resistivity.

Another activator or sensitizer embodiment of the present inventioncomprises a single layer of an oriented polymeric photoconductor, suchas a terpolymer of N- pentenylcarbazole, N-hexenylcarbazole, andpentene-l (mole ratio of 40:40:20, respectively) which has been stainedby, for example, 2,4,7-trinitro-9-fluorenone which activates thepolymeric photoconductor as well as serves to render it dichroic.

Other useful substrates than those previously mentioned are metallized(i.e.aluminum, and copper) polycarbonate and glass. Also NESA glass maybe used. Normally, it is preferred to use a transparent material whichwill neither depolarize nor change direction of the electric vector ofpolarized light. However, if desired, the substrate may serve topolarize incoming unpolarized light or change incoming polarized lightso that its electric vector is the proper direction for transmissionthrough the photoconductor.

A further modification of the present invention concerns a process inwhich a semi-transparent mirror is inserted in back of thephotoconductive element. That is, the exposure step now comprises whatcan be termed mirror dichroic reflex. With a semi-transparent mirrordisposed behind the photoconductive element during exposure, theincoming polarized light passes through the semi-transparent mirror andthe photoconductive element, but the depolarized light reflected fromthe document which is not absorbed by the photoconductive element isreflected back into the photoconductor by the highly reflective surfaceof the mirror. Thus, the reflected depolarized light not initiallyabsorbed by the photoconductive element will be reflected between themirror and the document, with additional light being absorbed by thephotoconductive element during each such reflection. This greatlyincreases the absorption efficiency of the photoconductive element witha consequent gain in the ultimate contrast between the print andbackground of the copy. Accordingly, this makes the performance of theprocess of the present invention relatively insensitive to thedepolarization factor of the document being copied. This gain is similarto that described in U.S. Pat. application Ser. No. 593,051 filed Nov.9, 1966. Preferably, if the metal forming the conductive surface of thesubstrate is made highly reflective, it may also serve as thesemi-transparent mirror.

Filters may be used with the exposing light source if it is necessary toeliminate wavelengths of light which would be absorbed by other than thedichroic entity or to confine the exposure light to those spectralregions which the element exhibits adequate dichroism.

The invention now will be further illustrated by the following examples,but it is to be understood that the invention is not restricted thereto.

EXAMPLE I A glass substrate was metallized with aluminum to a thicknesssuch that the optical transmission density was 1.0 (i.e.10%transmitting, the aluminum serving as a conductive electrode as well asa highly reflective mirror. A dichroic photoconductor of2-p-N,N-dimethylaminobenzylideneamino) 6 (pnitrobenzylideneamino[1,2-d:5,4-d']bisthiazole (described in US. Pat. 3,489,- 558) in drypowder form was lightly rubbed with an unidirectional motion to form athin coating having an optical density in the region of 0.20.6 withpolarized light oriented for absorption. A 10% by weight poly-N- vinylcarbazole in tetrahydrofuran was coated over the dichroicphotoconductive coating with a doctor blade set at a 5 mil wet gap, theresulting dry thickness being about 8-l0 microns.

The thus prepared photoconductive element was electrostatically chargedwith a Xerox Model D processor set at a negative --7000 volts to form auniform negative electrostatic charge. The charged element was broughtinto face-to-face contact with a document (black print on a whitebackground). Next, the document and charged photoconductive element wasexposed to polarized light from a 375 watt photo EBR flood lamp passingthrough a light polarizing film (supplied by Bausch and Lomb, CatalogNo. 31-52-62-26) through the back of the eleconductor (see table below)was rubbed in a unidirection on the coating. Next, i /2% solution ofpoly-n-vinylcarbazole in benzene was coated on the meniscus coater witha sufiicient number of passes to form an approximately micron layer.

Following the reproduction procedure of Example I, except for thedichroic photoconductors and exposure conditions set forth in thefollowing table:

Distance from source, Time, Photoconductor Light source inches sec.

III 2,6-bis-[p-dimethylaminobenzylideneamino]-benzo[1,2-d,4,5-d]bihiazo1e 375 watt photofiood (Example I) 12 0, 4 IV2,6-bis-[p-dimethylaminoeinnamylideneaminolbenzo[1,2-d,4,5-d]b1sthiaz0le 40 watt incandescent 130. 3 V 2,6-bis-[p-dimethylarninocinnamylideneaminokbenzo[l,2-d,5,1,d]b1sthiazole 376 watt photofiood (Example I) 12 0. 4 VI 2,a-bis-[5-(-dimethylaminopheny1)penta-2,4-dl n yhd n ammol-benzo[1,2-d,5,4-d] 40watt incandescent 13 4 bisthiazole. t

ment with the dichroic photoconductor aligned for low absorptionrelative to the electric vector of the polarized light so that the lightwas essentially transmitted through the element. In striking the whitebackground of the document, it was depolarized, reflected back, andabsorbed. The exposure was for 0.4 seconds at a distance of 12 inches.Next, the document was separated from the photoconductive element andthe remaining electrostatic charges in the unexposed (print) areas wasde- 'veloped by cascading positively charged toner particles which wereattracted to the negative electrostatic charge pattern. The developedpattern was transferred to a copy sheet to yield a copy of the documentwhich had high print density, excellent contrast and only a faintbackground.

EXAMPLE II A glass substrate was coated with polyvinylidene chloride toform a thermoplastic coating. Next, a dichroic photoconductor of 2,4bis(p-N,N dimethylaminobenzyhdeneamino)-benzo[1,2-d:4,5-d]-bisthiazolewas rubbed in an unidirection on a film of poly-N-vinylcarbazoleactivated with 2% by weight of tetrachlorophthalic anhydride and carriedon a temporary polyethylene terephthalate substrate. On the poly-N-vinylcarbazole film was an aqueous solution of a methylvinylether-maleicanhydride copolymer and quaternary ammonium salt in equal parts byweight using a doctor blade set at a 1 /2 mil wet gap. Upon drying, thethickness of this conductive coating was about 2-3 microns. To completethe photoconductive element, the polyvinylidene chloride coating on theglass substrate was brought into contrast with the conductive coating onthe poly-N-vinylcarbazole and laminated thereto by heating to about 100C. After cooling, the polyethylene terephthalate temporary substrate wasseparated to leave the finished photoconductive element.

This photoconductive element was used for reproducing a copy of adocument in the same manner as Example 1 except that the light sourcewas a 40 watt incandescent lamp and the element was 12 inches from thelamp when exposed for 1 second. The copy had high print density and goodcontrast with fair background.

When the electric vector of the incoming polarized light was rotated 90so that initially the light was uniformly adsorbed by thephotoconductive element, and the rest of the procedure remained thesame, the copy produced had very poor contrast and high background, andwas totally unsatisfactory.

EXAMPLES III-VI A film of cellulose triacetate metallized with aluminumhaving an optical density of 0.8 was coated with an aqueous solution ofa methylvinylether-maleic anhydride copolymer and quartenary ammoniumsalt in equal parts by weight using a meniscus coater. Upon drying, thecoating was about 2-3 microns thick. A dichroic photo- The copiesreproduced had high print density and good contrast with only faintbackground.

When the electric vector of the incoming polarized light was rotated sothat initially the light was uniformly absorbed by the photoconductiveelements and the rest of the procedure remained the same, the copiesproduced had very poor contrast, high background and were totallyunsatisfactory.

EXAMPLE VII A glass substrate was metallized with aluminum to athickness such that the optical density was 1.0 10% transmitting). Asolution of 10% by weight of poly-N- vinyl-carbazole in tetrahydrofuranwas coated with a doctor blade set at a 5 mil wet gap, the resulting drythickness being about 810 microns. Next, a dichroic photoconductor of2,6-bis-[p-dimethylaminobenzylideneamino]- benzo[l,2-d,5,4-d]bisthiazolein dry powder form was lightly rubbed with an unidirectional motion orin the long direction of the substrate to form a thin coating having anoptical density in the region of 0.2-0.6 with polarized light orientedfor absorption.

Following the reproduction procedure of Example I, except that the XeroxModel D processor was set at a positive +7000 volts and that the lightsource was a 40 watt incandescent lamp spaced at a distance of 12 inchesfrom the above prepared photoconductive element, and the exposure was 1second, a copy of a document was reproduced and it had high printdensity, good contrast, and only a faint background.

When the electric vector of the incoming polarized light was rotated 90so that initially the light was uniformly absorbed by thephotoconductive element, and the rest of the procedure remained thesame, the copy produced had very poor contrast and high background, andwas totally unsatisfactory.

EXAMPLE VIII For determining the dichroic photodecay ratio ofphotoconductive elements and, in addition, the measuring difference insurface potential of the photoconductive elements is exposed to stronglyabsonbed polarized light and weakly absorbed polarized light, thefollowing described electrometer was used and serves as a simulatedreproduction process. The exposures are over a period of time and thisgives a measure of the latitude of the photoconductive elements.

The electrometer comprises an electrostatic corona discharge chargingunit set at a potential of 6000 volts and a Model 566 Charge Amplifier(manufactured by Kistler) having a transparent NESA glass probe. Arotatable photoconductive element holder on a pivoted arm capable ofmoving the photoconductive element holder from the charging unit to infront of the transparent glass probe. For exposure of thephotoconductive element, the electrometer has a watt tungsten lamp witha filter holder disposed adjacent the lamp. In optical contact with the1 1 holder is a light tube which, in turn, is attached to thetransparent glass probe. On the back side of the glass probe is anH-sheet polarizer (manufactured by Polaroid). For observing themeasurements of the electrometer, the charge amplifier is connected to aType 535 Oscilloscope with a 53/ 54C Plug-In unit.

In operation, a photoconductive element sample is placed in the holderwith the photoconductive surface facing out of the holder and aligned,relative to the electric vector of the polarized light, to only weaklyabsorb the light. The arm is moved to place the sample in front of thecharging unit and is given an uniform electrostatic charge. Next, thecharged sample is moved in front of the transparent probe and exposed topolarized light from the lamp passing through the polarized light. Theexposure is observed on the oscilloscope as a trace moving from left toright. Depending on the dissipation of the electrostatic charge on thephotoconductive element, the trace also curves downwardly. The exposureis retained until the trace reaches the right side of the oscilloscope.This is a measure of the photoconductive elements response to polarizedlight, which it only weakly absorbed. Now, the element is rotated 90 andthe arm is moved back to the charging unit for uniform charging of theelement. After charging, the arm is again moved to the transparent probeand again exposed to polarized light, only this time the element isaligned to strongly absorb the light. During exposure, another tracemoves from left to right across the oscilloscope. This trace will curvedownward substantially and the former curve will have very littledownward movement if the element is a good dichroic photoconductor. Theface of the oscilloscope has a scale which permits calibrating thecurves which are a plot of surface potential vs. exposure time. APolaroid land camera is mounted on the oscilloscope and double exposurepictures or oscillograms of the traces are taken of each dichroicphotodecay ratio. These curves also yield the photoconductive speed T /2of each element.

The following table lists the properties of seven photoconductiveelements which were prepared by coating a poly-N-vinyl carbazole intetrahydrofuran solution on polyethylene tcrephthalate substratecarrying a conductive layer of a quaternary ammonium salt in polyvinylalcohol. To the poly-N-vinylcarbazole layer, the respectiwe dichroicphotoconductor was applied in powder form with an unidirectional wipingmotion. The thus prepared elements were tested on the above-describedelectrometer with the traces on the oscilloscope being photographed. Thedichroic ratio is also given and was measured on a Cary Model 14spectrophotometer. The filters are Balzers #2 and #3 filters, having xmax, respectively, of 4500 A and 4800 A.

tungsten and 22 with a #3 filter. The dichroic photodecay ratio of thephotoconductors of Examples III, IV, V, as measured by the above testingprocedure to full tungsten, was l8, l0, and 10, respectively.

EXAMPLE XV An aqueous solution of polyvinyl alcohol was coated on analuminum by an unidirectional wiping to form a thin oriented coating.This coating was then stained with a 1% iodine solution in acetone againwiping in the same unidirection. Next, a 10% polyvinylcarbazole solutionin tetrahydrofuran was coated on the stained polyvinylalcohol with adoctor blade set at a 5 mil wet gap. When evaluated with theabove-described electrometer, it had a dichroic photodecay ratio of 3.0.Accordingly, iodine in an oriented configuration is useful as a dichroicsensitizer in the dichroic reflex process of the present invention.

EXAMPLE XVI Light source: watt incandescent Distance from source: 12inches Time: 1.0 sec.

The above-described document was then copied on a Xerox 720 and a visualcomparison made. The Xerox 720 had good contrast between the black andwhite areas but there was essentially no grades of half tonereproduction. Instead, the half tone areas reproduced as continuouslight black areas.

Conversely, the copy reproduced by the dichroic reflex process of thepresent invention had excellent contrast between the black and whiteareas and good reproduction of the half tone areas.

While the invention has been particularly shown and described withreference to preferred embodiments thereof, it will be understood bythose skilled in the art that variations in form may be made thereinwithout departing from the spirit and scope of the invention. Forexample, the dichroic reflex process and photoconductive element of thepresent invention may be employed in persistent Diehroic photodecayratio Optical Full Per- #2 Ier- #3 Perdiehroic tungcent filcent filcentDichrole photoconductor ratio sten eff. ter etf. ter e11.

VIIL...2-(p-N,N-dimethylaminobenzylideneamino)-6-(p-nltrobenzylideneamino)-benzo[1,2-11.9 5 42 6 4G 5. 5 57 d,5,4-d]bisthiazole. IX2,G-l%is(p-N,N-dirnethylaminobenzylldeneamino)-4-methylbenzp[1,2-d,5,4-d]blsthia6.7 3.5 52 3 45 4. 5 67 zo e. X2,7-bis-(p-N,N-dirnethylamlnobenzylideneamino)-benzo[1,2,d,4,3d]bisth1azole8.3 4 48 2 24 2. 5 30 XI2,6diis-(p-N,N-dimethylaminobenzylideneamino)-4-ehlorobenzo[1,2d,5,4-d]blsthia-8. 3 6. 5 78 4 48 6 72 zo e. XII-..2,7-bis(p-N,N-dimethylaminobenzylldeneamino)-4-chlorobenzo[1,2-d,3,4-d1bisthia-1.4 1 1. 35 96 1. 35 96 z e. XIII.-."2,6-kbls-(Er-N,N-dimethylamlnobenzylideneamino)4-methoxybenzol1,2-d,5,4-d]bls-8. 9 5. 5 G2 5 G0 5 t iazo e. XIV2,7-bis(p-N,N-dimethylaminobenzylideneamino)-4-methoxybenzo[1,2-d,3,4d']bis-2. 58 2. 5 93 2. 5 93 tlnazole.

All of the above compounds are useful as dichroic photoconductors inphotoconductive elements for the dichroic reflex process of the presentinvention.

As evidence that the photodecay ratio will vary depending upon themethod of fabrication, the dichroic photoconductor of Example XIII wasprepared according to the procedure of Examples III-VI. The element thuselectrophotographic methods, such as that disclosed in US. Pat.2,845,348 or any other method where the photoconductor is exposed beforecharging. Also, the dichroic reflex process and photoconductive elementof the present invention can be used in conjunction with charge transfertechniques, such as disclosed in US. Pat. 2,825,814. As a furtherexample, the photoconductive element can be prepared had a dichroicphotodecay ratio of 19 with full fabricated with a non-conductivesubstrate and the charging of the element can be accomplished by dualcorona, such as described in U.S. Pat. 2,922,883.

What is claimed is:

*1. A photoconductive element, suitable for use in electrophotographicreflex copying, comprising a dichroic material oriented in anunidirection to have preferred maximum absorption and transmission axes,and exhibiting photoconductive dichroism, said dichroic material beingcapable of causing conductivity in those areas of the photoconductiveelement which are exposed to light having an electric vector ofsuflicient magnitude and in a direction parallel to said absorptionaxis.

2. The photoconductive element of claim 1 wherein said element containsa semitransparent mirror on the side of the absorption axis opposite tothe side of the element to be brought into contact with a document, thereflective surface of said mirror facing towards the side of the elementto be brought into contact with the document.

3. The photoconductive element of claim 2 wherein said mirror is formedof a conductive metal and also serves as an electrode.

4. The photoconductive element of claim 1 wherein said element comprisesa transparent substrate having disposed thereon a thin layer of adichroic photoconductor and a thicker essentially transparent chargetransport layer, said photoconductive layer and said charge transportlayer being in contact with each other, the composite of said layersbeing of suflicient resistivity to support an electrostatic charge inthe dark, said dichroic photoconductive layer being oriented in anunidirection to provide said preferred absorption axis, the lightabsorption spectra of said charge transport layer being essentiallyoutside the light absorption spectra of said dichroic photoconductivelayer.

5. The photoconductive element of claim 4 wherein said substrate of theelement carries a semitransparent mirror with its reflective surfacefacing said preferred absorption axis.

6. The photoconductive element of claim 5 wherein said semitransparentmirror is formed of a conductive material and also serves as anelectrode.

7. A photoconductive element of claim 1 wherein said element comprises atransparent substrate having disposed thereon a layer of a polymericmaterial oriented in an unidirection, a layer of an essentiallytransparent photoconductor, and a material at the interface of saidoriented polymeric layer and said photoconductive layer capable ofrendering the oriented layer dichroic to provide said preferredabsorption axis and of serving as an activator for the photoconductor.

8. The element of claim 7 wherein said material at said interface beingcapable of forming a charge transfer complex with said photoconductor.

9. The photoconductive element of claim 7 wherein said substrate of theelement carries a semitransparent mirror with the reflective surfacefacing said preferred absorption axis.

10. The photoconductive element of claim 9 wherein said semitransparentmirror is formed of a conductive material and also serves as anelectrode.

11. An electrophotographic process for the production of reflex copiescomprising:

positioning a document, having a pattern of light absorbing areas andlight depolarizing and reflecting areas, adjacent a photoconductiveelement which exhibits photoconductive dichroism and has a preferredabsorption axis;

uniformly exposing through the photoconductive element with polarizedlight whose electric vector is essentially normal to the absorption axisof said photoconductive element, said polarized light being depolarizedby those depolarizing and reflecting areas of the document and, in beingreflected back into said element, containing light with an electricvector 14 parallel to the absorption axis of said element so as torender those areas of the element conductive, said conductive areas ofsaid element being capable of transporting an electrostatic charge tocause formation of an electrostatic charge pattern corresponding to thepattern of said document.

12. The process of claim 11 wherein the photoconductive element isuniformly electrostatically charged prior to said exposure to polarizedlight.

13. The process of claim 12. wherein after said exposure, said documentis removed from photoconductive element and the electrostatic chargepattern thus formed is developed.

14. The process of claim 13 wherein the electrostatic charge pattern isdeveloped on the photoconductive element and the developed patterntransferred to a copy sheet.

15. The process of claim 14 wherein the photoconductive element iscleaned to remove any residual of the developed pattern and is reusedfor the reproduction of another copy.

16. In an electrophotog-raphic process, the step comprising:

exposing a photoconductive element, exhibiting photoconductive dichroismand having a preferred absorption axis, to a light pattern resultingfrom reflection of light from a document in contact with thephotoconductive element and in accordance with the pattern of thedocument, said light pattern containing light with an electric vectorparallel to the absorption axis of said element so as to renderconductive those areas of the element corresponding the light pattern,said light prior to reflection from the document having a transmissiondirection opposite to the reflected light pattern and being polarizedlight with an electric vector essentially normal to the absorption axisof said photoconductive element as it passes the axis so that saidpolarized light is essentially transmitted by the photoconductiveelement.

17. The process of claim 16 wherein the photoconductive element containsa semitransparent mirror on the side of the absorption axis opposite tothe side of the element in contact with the document whereby saidpolarized light is transmitted through the mirror but each portion ofthe reflected light pattern not absorbed at the absorption axis isreflected back and forth between the mirror and the document therebyincreasing the total light absorbed by the photoconductive element.

18. The process of claim 17 wherein said mirror is formed of aconductive metal and also serves as an electrode.

'19. The process of claim 16 wherein said photoconductor comprises atransparent substrate having disposed thereon a thin layer of a dichroicphotoconductor and a thicker charge transport layer, saidphotoconductive layer and said charge transport layer being in contactwith each other, the composite of said layers being of sutficientresistivity to support an electrostatic charge in the dark, saiddichroic photoconductive layer being oriented in an unidirection toprovide said preferred absorption axis, and wherein the wavelengths ofsaid polarized light are essentially outside the wavelengths ofabsorption by said charge transport layer.

20. The process of claim 19 wherein said substrate of thephotoconductive element carries a semitransparent mirror with itsreflective surface facing said preferred absorption axis.

21. The process of claim 20 wherein said semitransparent mirror isformed of a conductive metal and also serves as an electrode.

22. The process of claim 16 wherein said photoconduc tive elementcomprises a transparent substrate having dis posed thereon a layer of apolymeric material oriented in an unidirection, a layer of aphotoconductor absorbing essentially outside the wavelengths of thepolarized light,

15 and a material at the interface of said oriented polymeric layer andsaid photoconduetor layer capable of rendering the oriented layerdichroic to provide a preferred absorption axis and serving as anactivator for the photoconductor.

23. The process of claim 22 wherein said substrate of thephotoconductive element carries a semitransparent mirror with itsreflective surface facing said preferred absorption axis.

24. The process of claim 23 wherein said semitransparent mirror isformed of a conductive metal and also serves as an electrode.

References Cited UNITED STATES PATENTS 16 3,113,022 12/1963 Cassiers eta1. 961.5 3,165,405 1/1965 Hoesteney 961.7

3,271,145 9/1966 Robinson 96-1 3,272,626 9/1966 Shinn 96-1 3,278,30210/1966 Gundlach 96-1 3,287,122 11/1966 Hoegl 96-1.5 3,312,547 4/1967Levy 961.5

3,317,317 5/1967 Clark 961.4

CHARLES E. VAN HORN, Primary Examiner US. Cl. X.R.

